Pretty table printing

[soutaro]

steep stats now support table and csv printing modes. Users can specify the mode through --format options.

$ steep stats --format=table
 Target  File                                         Status   Typed calls  Untyped calls  All calls  Typed %
--------------------------------------------------------------------------------------------------------------
 test    argument_type_mismatch.rb                    success            0              0          1       0%
 test    block_body_type_mismatch.rb                  success            1              0          2      50%
 test    block_type_mismatch.rb                       success            0              2          3       0%
 test    break_type_mismatch.rb                       success            1              0          1     100%

The default mode depends on STDOUT.tty?. table mode is used if it's true and csv mode is used if it's false. This assumes a workflow to run steep stats command behind a pipe and save the result as a CSV file for further visualization/review/processing. This PR won't break the workflow.

[ybiquitous]

[IMO] Since ArgumentError occurs on an invalid --log-level value, it seems kind to me to raise ArgumentError also on an invalid --format value.

Error:

$ bundle exec steep stats --log-level=fata
#<ArgumentError: invalid log level: data>

No error:

$ bundle exec steep stats --log-level=fatal --format=tabl
--format option is ignored because it has unknown value: `tabl`
# Calculating stats:

 Target  File                                             Status   Typed calls  Untyped calls  All calls  Typed %
------------------------------------------------------------------------------------------------------------------
...

[soutaro]

Good point! Will do. 💪